The present invention relates to planar illumination devices and planar light-sources and, more particularly, to single-sided or double-sided planar light-source devices suitable for thin advertisement signboards, display units, planar illumination instruments or the like, and an illumination apparatus for illuminating various instruments such as liquid-crystal display instruments or the like having incorporated therein the planar light-source devices.
The main current of a particularly large advertisement signboard or illumination apparatus, which has conventionally been put into practical use, has such a construction that one or more fluorescent lamps are arranged within a housing, and a diffusion plate is arranged in spaced relation to the fluorescent lamps by a suitable distance. In such apparatus, however, if the distance between the fluorescent lamps and the diffusion plate is insufficient, emission lines of the fluorescent lamps called "lamp images" can be viewed, so that the depth of the apparatus must inevitably increase in order to secure uniformity of brightness. If the depth is decreased in such apparatus, diffusion performance of the diffusion plate cannot but increase. Since, however, this causes a reduction in light-ray transmittance, the number of fluorescent lamps cannot but increase in order to maintain the same brightness. Thus, there arise such problems as countermeasures for an increase in consumptive electric power, a rise in temperature and so on.
In order to solve these problems, many proposals have conventionally been made (Japanese Utility Model Publication No. SHO 42-18278, Japanese Patent Provisional Publication No. SHO 55-15126, Japanese Patent Provisional Publication No. SHO 55-133008 Japanese Utility Model Provisional Publication No. SHO 56-35667 and Japanese Patent Provisional Publication No. SHO 59-22493). Since, however, these proposals are chiefly such that an upper portion immediately above a light source is shielded to cause the emission lines to disappear, uniformity is made in conformity with a portion dark in face. Thus, this is not preferable from the viewpoint of utilization efficiency of a quantity of light.
Further, as being thinkable in principle, it is adapted that approximation of a point source is used to arranged a lamp at a focus of a convex lens, and a light passing through the convex lens is brought to a parallel light, and it is possible to incorporate a Fresnel lens having such function in the light source. Since, however, the fluorescent lamp is not the point source, reproducibility of the principle is deteriorate so that it is the actual circumstances that the fluorescent lamp cannot be put to practical use.